The present invention generally relates to image transfer technology and, more particularly, to an apparatus and method for reducing contamination of image transfer surfaces of an image transfer device during the printing process, and an image transfer device having the apparatus.
As used herein, the term “image transfer device” generally refers to all types of devices used for creating and/or transferring an image in a liquid electrophotographic process, including laser printers, copiers, facsimiles, and the like.
In a liquid electrophotographic (LEP) printer, the surface of a photoconducting material (i.e., a photoreceptor) is charged to a substantially uniform potential so as to sensitize the surface. An electrostatic latent image is created on the surface of the photoconducting material by selectively exposing areas of the photoconductor surface to a light image of the original document being reproduced. A difference in electrostatic charge density is created between the areas on the photoconductor surface exposed and unexposed to light. In LEP, the photoconductor surface is initially charged to approximately ±1000 Volts, with the exposed photoconductor surface discharged to approximately ±50 Volts.
The electrostatic latent image on the photoconductor surface is developed into a visible image using developer liquid, which is a mixture of solid electrostatic toners or pigments dispersed in a carrier liquid serving as a solvent (referred to herein as “imaging oil”). The carrier liquid is usually insulative. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and toner. The photoconductor surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For LEP printers, the preferred embodiment is that the photoconductor surface and toner have the same polarity.
A sheet of paper or other medium is passed close to the photoconductor surface, which may be in the form of a rotating drum or a continuous belt, transferring the toner from the photoconductor surface onto the paper in the pattern of the image developed on the photoconductor surface. The transfer of the toner may be an electrostatic transfer, as when the sheet has an electric charge opposite that of the toner, or may be a heat transfer, as when a heated transfer roller is used, or a combination of electrostatic and heat transfer. In some printer embodiments, the toner may first be transferred from the photoconductor surface to an intermediate transfer medium, and then from the intermediate transfer medium to a sheet of paper. After the toner transfer has occurred, the photoconductor surface is cleaned and recharged in preparation for the printing of a subsequent image.
Charging of the photoconductor surface may be accomplished using any of several types of charging devices, such as a corotron (a corona wire having a DC voltage and an electrostatic shield), a dicorotron (a glass covered corona wire with AC voltage, and electrostatic shield with DC voltage, and an insulating housing), a scorotron (a corotron with an added biased conducting grid), a discorotron (a dicorotron with an added biased conducting strip), a pin scorotron (a corona pin array housing a high voltage and a biased conducting grid), or a charge roller that contacts the photoconductor surface.
Each of these charging devices generate ozone (O3), and nitric oxides (NOX) in varying amounts, which if present in sufficient quantities, must be vented and filtered from the image transfer device. The high voltages and currents required for corona discharge devices tend to generate greater amounts of ozone and nitric oxides, while contact charging devices tend to generate smaller amounts of ozone and nitric oxides.
An active flow of air through the image transfer device may be provided to ventilate and filter ozone and/or nitric oxides from the image transfer device. In addition, an active flow of air through the image transfer device may also be provided for controlling heat build-up inside the device. In other instances, an active flow of air may be spontaneously created due to factors including high speed movement of photoconductor surface or other surfaces, and convective currents caused by heat generated within the image transfer device.
Although an active airflow through the image transfer device is sometimes required or desired for ventilation and/or cooling purposes, airflow past the photoconductor surface is problematic in long term use of the photoconductor surface. In particular, active airflow is problematic because the airflow evaporates the submicron layer of imaging oil on the photoconductor surface and entrains oil vapors present above the oil layer, thereby effectively thinning the oil layer. The remaining oil layer includes residual materials such as charge directors and other dissolved ink components that have high molecular weight and do not easily evaporate. The thinned oil layer provides reduced buffering of the molecules of residual material against ion bombardment, UV exposure and ozone penetration caused by the charging device. Therefore, the residual materials in the oil layer are more likely to react and polymerize on the photoconductor surface. Additionally, the dissolved residual material in the thinned oil layer is much closer to or beyond its solubility limit. This increases the chance for dissolved residual materials to drop out of solution and polymerize on the photoconductor surface. In the case of contact charging devices, the residual materials and polymers thereof may be forcibly pressed against the photoconductor surface, thereby increasing the amount and rate of contamination of the photoconductor surface. During the printing process, and particularly after the photoconductor surface is cleaned in preparation for a subsequent printing cycle, it is desirable that the photoconductor surface is free of residual materials from previous printing cycles, such as toner, charge directors and other dissolved materials in the imaging oil. However, effectively cleaning the photoconductor surface of all residual materials is very difficult, and some amount of residual material inevitably remains on the photoconductor surface. Due to the energy imparted by the charging device during the charging process, and the highly reactive ozone and nitric oxides generated by the charging device, over time molecules of the residual materials on the photoconductor surface react and polymerize to generate sticky materials that slowly but steadily form a film or coating on the photoconductor surface. The filming of the photoconductor surface eliminates the ability to either form latent images of small dots on the photoconductor surface, or to transfer small dots from the photoconductor surface to paper. As filming of the photoconductor surface increases over time, the print quality of subsequently printed images is reduced, and the useful life of the photoconductor surface is shortened. The filming problem is often referred to as old photoconductor syndrome (OPS). Therefore, there is a need for an apparatus or method to lessen or eliminate polymerization of the residual materials and the resulting filming of the photoconductor surface.